The invention relates generally to constant temperature, i.e., fine wire/thin film or hot-wire/hot-film, anemometers and more specifically to a constant temperature fine wire or film sensor driven by a pulse width modulated digital control loop.
The accepted manner of sensing fluid flow velocities, particularly velocities of air, comprehends the use a short, fine wire or thin film sensor disposed in the fluid flow. An electrical current is passed through the wire or film, heating it and, through such heating, varying the resistance of the wire or film. If the electrical energy provided to the wire or film is equal to the energy transferred by the wire or film by virtue of the fluid flow around it, equilibrium is achieved. During such equilibrium, the energy provided to the wire or film to maintain its temperature is a function of and may be utilized to infer the fluid velocity.
Presently available constant temperature anemometers utilize a Wheatstone bridge in conjunction with a high gain solid state feedback amplifier. See Comte-Bellot, G., 1976 "Hot-Wire Anemometry," Annual Review of Fluid Mechanics, pp. 209-231, Annual Reviews, Inc. Palo Alto, Calif., Blackwelder, R. F., 1981, Hot-Wire and Hot Film Anemometers, Academic Press, Inc., New York, N.Y. and Perry, A. E., 1982, Hot-Wire Anemometry, Clarendon Press Oxford University Press, New York, N.Y.
While the advent of high gain, solid state electronics in this field greatly improved the accuracy and expanded fine wire and thin film anemometry applications, operational problems and limits on applications still exist. For example, the signal to noise ratio, the frequency response and the oscillatory instability of such amplifiers as well as the use of the basic Wheatstone bridge circuit still place operational limits on fine-wire or thin film anemometers. The accuracy of measurements taken with such devices is affected by such considerations.
Significant patented technology in this measurement field exists. For example, U.S. Pat. No. 3,905,230 which utilizes a thermoresistive sensor in one leg of a Wheatstone bridge and a pulse generator which sequentially provides a first pulse and a second pulse, the second pulse being provided before the sensor has returned to thermal equilibrium. The flow rate is determined by measuring the resistance of the sensor during the pulses. U.S. Pat. No. 4,334,186 teaches a hot wire constant temperature air flow sensor which includes an ambient sensing and compensating element. A pulse train having pulse widths corresponding to differences between the voltage drops across the hot wire sensor and the temperature compensating element interrupts the current supply to the hot wire.
U.S. Pat. No. 4,565,091 again teaches a pair of sensing elements disposed in a Wheatstone bridge. One of the elements is heated in order to achieve air flow measurement and the other element constitutes an ambient temperature measuring means. The outputs of the two sensors are provided to the inputs of a comparator which drives the reset input of a flip-flop. The set input of the flip-flop is driven through an AND gate by either the signal from a burning off controller or an initializing pulse. U.S. Pat. No. 4,794,794 discloses a thermal anemometer including a constant temperature bridge excitation circuit, multiple signal comparators and a tapped resistance divider which provides a linearized digital output and a linear analog output.
U.S. Pat. No. 4,807,151 utilizes a mass air flow sensor and bridge circuit. The bridge circuit generates a first mass flow signal having rate and temperature related errors and resistances which provide respective signals for air temperature correction and mass flow rate correction.
U.S. Pat. No. 4,934,188 utilizes a modified Wheatstone bridge circuit having a heated sensing element for sensing flow rate and a second sensing element for sensing air temperature. The output of the Wheatstone bridge is provided to a comparator and a voltage to frequency converter. Improved responsiveness of the temperature sensing element is claimed. U.S. Pat. No. 5,074,147 discloses a flow rate measuring apparatus which maintains constant voltage or constant power to a hot wire or hot film sensor. An operational amplifier disposed in a negative feedback loop drives the sensor. Finally, U.S. Pat. No. 5,493,906 teaches another constant temperature anemometer which utilizes a Wheatstone bridge and which includes a pair of operational amplifiers, the second of which provides a correcting offset voltage which is claimed to improve the stability and frequency band width of the device.
From the foregoing, it is apparent that significant prior art exists but that improvements in the art of constant temperature hot wire anemometry are achievable.